1. Field of Invention
This invention relates generally to differential-pressure transducers of the capacitive type, and more particularly to a transducer having a pair of diaphragms, one responsive to an applied high pressure and the other to an applied low pressure, each diaphragm functioning as the movable electrode of a variable capacitor.
2. Status of Prior Art
It is known in industrial process control systems to use differential-pressure (D-P) transducers for flow measurement. Thus to determine the flow rate of a process fluid (liquid or gas) in a pipeline, an aperture plate is interposed therein and a differential pressure transducer acts to sense the degree of pressure drop developed between opposite sides of the aperture plate. This drop is directly related to flow rate. (The pressure drop is proportional to the square of the flow rate.)
One simple form of differential pressure transducer of the capacitive type consists of a case divided by an electrically conductive diaphragm into a pair of chambers, each having a stationary electrode mounted therein to define with the conductive diaphragm a capacitor whose value varies as the diaphragm is deflected. Low fluid pressure from the aperture plate is applied to one chamber and high pressure to the other.
In this known form of D-P transducer, the conductive diaphragm and the stationary electrodes on either side thereof form a three-terminal capacitive potentiometer in which the diaphragm is the variable element. By arranging this element in a bridge circuit excited by a voltage of, say, 10 kHz, the capacitance changes caused by diaphragm deflection unbalance the bridge to produce a 10 kHz output whose amplitude is directly proportional to pressure.
German patent publication No. 23 64 027 discloses a D-P transducer consisting of a hollow metal cylinder having an insulating body therein, with diaphragms welded to either end to enclose interconnected cavities filled with an incompressible fluid. These diaphragms form capacitors in conjunction with film electrodes mounted on opposing faces of the insulating body. Because the metal diaphragms carry an electrical potential, sparking may occur between the diaphragms and grounded parts adjacent thereto. Such sparking represents a hazard should the fluid being metered be ignitable.
A similar D-P transducer arrangement using metal diaphragms is disclosed in U.S. Pat. No. 4,398,194 to Johnson in which the transducer is formed by a pair of opposing sensing chambers bounded by these diaphragms which in conjunction with conductive layers coated on an insulating body form variable capacitors. The cavities created between the diaphragms and the body are filled with an incompressible dielectric fluid and are interconnected by a single duct going through the body. Also of prior art background interest are the patents to Paquin et al., U.S. Pat. No. 4,301,492 and Bell et al., U.S. Pat. No. 4,458,537.
The prior art reference of greatest interest is the Orlowski et al. U.S. Pat. No. 4,531,415. This patent discloses a D-P transducer in which the diaphragms are made of ceramic or other insulating corrosion-resistant material and have on their inner surfaces film electrodes which face the end surfaces of an insulating body on which stationary electrodes are formed. Hence the outer surfaces of the diaphragms can be exposed to either corrosive or non-corrosive process fluids.
In Orlowski et al., the insulating body together with the diaphragms define a pair of cavities which are filled with silicone oil or other dielectric fluid via a single filling duct in the body. In order to prevent damage to the diaphragms as a result of overdrive caused by excessively large input pressure differences, deflection of each diaphragm is limited by raised supports on the surfaces of the insulating body facing the diaphragm. Groove-like recesses are formed in these supports which are connected to the filling duct in the body, so that in the case of an abrupt pressure variation, the oil can be quickly distributed, thereby substantially reducing the response time of the transducer.